FMF - Friends of Minerals Forum, discussion and message board

[Pierre Joubert]

[Pete Richards]

[Harjo]

Now you should be able to watch the video ;-)

[vic rzonca]

Faden? Found on social media.

[Harjo]

Definitely not a Gwindel. This could be a faden crystal, but it looks more like a double terminated "rehealed" crystal. A crystal comes loose from the pocket wall and subsequently crystallisation continues (when the conditions are still favourable for crystallisation) or resumes (when the conditions become favourable again) on the fracture.

[GneissWare]

Here is an example of in situ fadens from near Quartzsite, Arizona.

[Philippe Durand]

very interesting thread; thank you for your teaching .

[Jordi Fabre]

Is a great pleasure see discussions from FMF "externalized" in so interesting printed papers like the A.G.A.B. bulletin.

The A.G.A.B. is the prestigious association of the Belgium collectors/geologists and as you can see in the images below, there Mr. Roger Warin took what was discussed in this thread and improved it with their own thoughts.

Mr. Roger Warin and the A.G.A.B. were so kind to allows me reproduce this article here as well as kind of digest from Mr. Warin written in English language (the bulletin is published in French language) in order to make their thoughts more comprehensible for the English speakers.

I believe is a great synergy what Mr. Warin did and I encourage all the FMFers to continue this scientific and so interesting discussion.

I love science! ;-)

FADEN QUARTZ - A personal opinion

Differently inspired and in opposition to the notion of a pre-existing chalcedony soul in a hydrothermal liquid and disappointed by the classical hypothesis, while remaining sensitive to the Pete Richards’s remarks of especially on the very low frequency of the phenomenon generating the Faden quartz, I suggest that if the process of the origin of these Faden crystals is so difficult to specify, it is that one observes only the crystal. One forgets to consider the initial medium which gives birth to these “quartz à âme” (in French). As noted by Pete Richards, these environmental conditions are not frequent, and therefore the environment must be special.

In what natural laboratory are these Faden quartz synthesized? Of course, in an alpine environment, or assimilated. That is to say, where tectonics induces high temperatures and pressures conditions in which very hot hydrothermal solutions circulate. Under these parameters of environment, we must forget the notion that we have a solvent such "liquid water" at room temperature. This insipid liquid, indispensable to life, becomes a chemical reagent at high temperatures. The notions of pH differ (the corresponding theoretical notions are known) and silica SiO2 can dissolve in this liquid, which seems unlikely at ordinary temperature (for example, sand in sea). Formally, by "adding" H2O to a molecule of SiO2, H2SiO3 and H4SiO4 are obtained. The molecule H4SiO4 or Si(OH)4 is silicic acid. In a hydrothermal phase this product crosslinks, that is to say it polymerizes in all three directions to an amorphous phase of silica gel. It is bulk crystallization or a mass polymerization is carried out. This phase is more stable than imagined, so it does not lose its water below 300 °C under pressure. On the other hand, it can crystallize by abandoning crystals of a polymorphic phase of quartz, as a function of the temperature which controls the crystallization. For temperatures below 450 ° C, we obtain the quartz (alpha-quartz, which we know). Much higher temperature values than those of alpine environments would give rise to cristobalite, for example.

The composition of silica gel is based on a distribution of associated Si(OH)4 molecules and water to form [Si(OH)4]x. n H2O. This assembly is crosslinked forming a condensation product (bulk polymerization) in all three directions. Its structure is composed of SiO4 units joined by internal siloxane bridges (Si-O-Si). It is an amorphous phase. Siloxane bridges have been created by condensation of two intermediate silanol groups with loss of one molecule of water. Dry, the material is hard and porous. It is thus normally used as desiccant.
All Fadenquarz (in German) appear in an Alpine vug. This narrow cavity differs from a fault which it supposes in addition a transverse displacement of the walls. These crystals originate in an almost closed system where high temperatures and pressures prevail. This cavity is filled with a silica gel, in hydrothermal solution.

Under the effect of a large impact, for example an adiabatic expansion induced by the spacing of the walls of the diaclasis (the rupture of the box), a substantially linear ribbon appears between the two walls, consisting of individual nano-crystals (minute-crystals) of quartz. John Kashuba (Oregon) ask to cut thin sections of a specimen of Faden quartz, at the level of the core, according to his instructions (Figs 5-8). John Kashuba chose a thin section thickness of 100 μm instead of 30 μm to achieve deep microscopic vision. The observation is made in incident light to observe only the inclusions and not the crystal itself. John Kashuba concludes by saying that he sees a simple quartz crystal with a confusing network of transverse non-axial planes, filled with fluid inclusions / defects / negative crystals.

All the conditions being identical during this expansion, these microcrystals align substantially with each other (FIGS. 4, 5, 10, 15). Here we find the concept of chalcedony microcrystals proposed by Marco Campos-Venuti. A curvature will only appear if the shock wave is deflected within the silica gel. At this point, nothing favors the rapid crystallization of the gel, which has lost its internal tensions, but the shock wave is damped, the core no longer develops laterally. It is only after the amorphous mass of the rich (water saturated) silica gel enveloping the aligned nuclei that the core of silica gel continues to feed, for reasons of thermodynamic stability, the growth of several macroscopic monocrystals, which are often very clear. The existence of isolated Faden quartz, biterminated, proves that the attachment of the core at both ends is not necessary. This process is relatively rare because the feeder gel must be enclosed in a narrow jail, a sort of flattened box set between the two walls of the rock. This explanation also agrees with the parallel aggregates of Faden Quartz aligned in the vugs (or small crevices) (Figs 9 and 10).

This primordial crystallization rate is great, incorporating bubbles of fluids in the opening up of small crystal domains. In fact, these fluid inclusions would themselves originate from the crystallizing gel zone, rather than an exchange with the outside. Water and gas are released from the gel during crystallization. In practice, there would be a series of crystal lattices of substantially aligned quartz, since the flash of crystallization would not have allowed a position of equilibrium between all these micro-domains. Then, the growth of the crystal (or the crystal sequence) could resume normally, the initial stresses having disappeared. The value of this hypothesis lies in the fact that the linear germ ribbon is supported by the gel (viscous) and does not float in an aqueous (liquid) solution.

When we observe the set of substantially parallel crystal chains and perpendicular to the walls, it is also understood that these aggregates are derived from the crystallization of a mass of silica gel. A "fracture-migration" crystallization sequence does not seem consistent, whereas shocked silica gel can induce several columns of parallel Fadens in the mass. Even the texture of these aggregates is an argument in favor of our hypothesis.

In this proposed environmental context, we can think of other mechanisms of initiation of crystallization. Thus, the creation of a hydrodynamic cavitation would make it possible to induce nucleation and the direct production of crystals. This cavitation could be caused by the sudden drop in pressure, by a wave or by any impurity that strikes the silica gel. Let us not forget that the collapse of a vacuum bubble in a liquid creates a strong localized shock wave (corrosion of boat propellers).

In conclusion, we propose this new hypothesis of the origin of Faden quartz with a core based on the nature of the initial medium enclosed in a rocky pocket that receives a shock wave, either by the sudden opening or spacing of the original cavity by Tectonics, causing a sudden drop in pressure, or by another process. This medium is a silica gel, multi-crosslinked by bridges of the siloxane type and saturated with water under high temperatures and pressures.

Roger Warin

[Pierre Joubert]

Here are a few very small samples from the Western Cape.

[Tobi]

[Pierre Joubert]

[Tobi]

[Jordi Fabre]

In a kind of "pendulum's swing" this thread has come and gone. It was created here and it was picked up by the adorable mini-magazine MiniBul of the Belgian Mineralogical Society A.G.A.B. that picked it up and expanded it into an article, and now it comes back to FMF thanks to the permanent kindness of Mr. Roger Warin that has authorized us to reproduce it here and he even translated it to English language through his friend Christian Servais, PhD at the University of Liège.
I recommend to all the FMFers to read it and then return to impel the pendulum, this time (and again) from FMF ;-)

This is the introduction to this swing pendulum that Mr. Warin has done:
When we admire the minerals, we can’t prevent us to dream. The human mind wanders... But where do these remarkable crystals come from? Even the most modest specimens ask questions.
The pearls of the collections are often secreted by hydrothermal waters. But what is nourishing this liquid? Unusual temperatures and pressures prevail in this environment. All notions change. The rock crystals of the Alps inspire purity. Their origin was sometimes singular. They are born in hydrothermal conditions. I was led to generalize these origins. In both cases, the hydrothermal medium was relatively pure silica. The only differences are the pressures and temperatures of the hydrothermal system.
Amateur of meteorites, I also studied the Libyan Desert Glass and all these reflections have allowed me to suggest the important role of silica SiO2 "solutions" feeding crystals of quartz below 450 °C and of cristobalite and other allotropes dispersed in Libyan Desert Glass above 1500 °C.
Is in this spirit that I took the description of the growth of the various Faden Quartz.
To see it click on this link or in the image:

[Frei Hans]

In this faden quartz from Pakistan the faden seems to be broken. What happened?

[Roger Warin]

I see a shear that followed the first flash crystallization of the soul. As the gel is viscous, it would have kept in memory the previous situation prior to calm and normal crystallization in a second step.
A very slight sliding of the walls relative to each other (a few hundred microns < 1 cm) caused this fracture of the soul just after its precipitation. The ends are then welded. In a dense gel, there is no reason for the pieces of the soul to fall while in a liquid it would be immediate.
Your observation therefore supports my hypothesis that the hydrothermal medium is a viscous gel of SiO2.
Thank you for your comment.